Well, this is not a question, but a small guide for anyone wondering how to use root level domains on their subdomain WordPress multisite setup.
I'm using Amazon Lightsail instance, however, this can be adopted very easily for any VPS of your choice.
pre-requisites are obvious and an important A DNS record:
main/root domain: primary.com
secondary domain: secondary.com
secondary.com points to the same IP and that IP is attached to the same instance as the primary.com
(ie. you'll be making a new zone for secondary.com and A record points to the IP of primary.com)
Now, a small note about optional wildcard SSLs. If you want your 2 domains to have the flexibility of issuing subdomains from the WordPress backend itself, without the need to tinker CNAME, every time you add a new subdomain, then you are required to add some new A record DNS.
*.primary.com pointing to the IP of the instance.
*.secondary.com pointing to the IP of the instance.
Having said that, I have added notes in parenthesis for the following bash commands where you are required to add some additional params specifically for deploying wildcard SSLs.
Please note, wildcard certs are not necessary for domain mapping (neither primary nor secondary), but absolutely must if you have plans to use something like this sub1.primary.com, sub1.secondary.com, sub2.secondary.com in future. No harm in deploying wildcards in the first place, as you'll have the flexibility from the get-go.
Now navigate to SSH
site primary.com -wp -cache=on
site primary.com -ssl=on
(use the following instead if you need subdomains on primary.com
site primary.com -ssl=on -wildcard)
site primary.com -multisite-convert
site secondary.com -parked=primary.com
site secondary.com -ssl=on -root=primary.com
(use the following instead if you need subdomains on secondary.com
site secondary.com -ssl=on -wildcard -root=primary.com)
Now, add a new WordPress subdomain on your primary: Sites > Add New
Site URL: secondary.primary.com (I usually use the TLD itself as the subdomain for easy management, you can choose any, as we'll be changing this value again in next step)
Use any title and admin email of your choice then click Add.
Navigate to the newly added subdomain and change the Site URL from the subdomain to https://secondary.com
That's it, your secondary domain will resolve form WordPress multisite which is managed by your primary domain.